The present invention relates to an automatic gain controller and, more particularly, to an automatic gain controller operating at high speed.
An automatic gain controller is an apparatus for controlling the output signal of a receiver such that a constant amplitude is output at all times for subsequent circuitry regardless of the amplitude of the input signal. The amplitude of the input signal to the receiver may fluctuate due to the inhibitory factors of a transmission channel of radio or wire communication.
During communication, sudden unexpected inhibitory influences in the transmission channel often occur, such that the received signal cannot but vary according to certain abnormalities. Such signal variations are caused by the loss of the signal path due to changes in weather conditions such as clouds or rain, due to sudden changes in the atmosphere or ionosphere, or due to a change in the length of a transmission line. Unless these changes in the amplitude of the received signal are actively dealt with, many errors and other problems are generated when recovering information contained in the received signal.
When the amplitude of the received signal due to the inhibitory factors of a transmission line changes, an automatic gain controller detects amplitude changes in a received signal, to thereby control the gain of the receiver such that if signal strength diminishes, the automatic gain controller increases the receiver gain and, conversely, if signal strength rises, the automatic gain controller decreases the gain. Accordingly, the automatic gain controller controls the output signal of the receiver to have a constant amplitude at all times. This automatic gain controller is widely used for a variety of receivers in the fields of satellite communications, ground network communications, mobile communications, etc.
The conventional automatic gain controller receives a signal, and then detects a change of its amplitude to control the gain accordingly. Therefore, the automatic gain controller cannot expect the change in the received signal, and then, the automatic gain controller is required to operate at high speed to control the gain of the rceiver for outputting an accurate signal.
FIG. 1 is a circuit diagram illustrating a construction of a conventional automatic gain controller for controlling the gain of a receiving amplifier.
Referring to FIG. 1, a conventional automatic gain controller comprises a receiving amplifier 44, signal detecting means 41, time constant controlling means 42, and control voltage amplifying means 43. Receiving amplifier 44 amplifies a signal having various amplification factors and received via a receiving signal input S1 and output as a receiving signal output S2. In general, most receiving amplifiers 44 include an automatic gain control node for controlling their overall gain, to prevent the output from being changed due to varying losses in transmission lines as described above. A gain control voltage S3 is supplied via the automatic gain control node so that output signal S2 can be stabilized without being influenced by a change of the input signal S1.
Signal detecting means 41 receives a signal from receiving signal output S2 of receiving amplifier 44 or from some other source, and then converts the signal into a corresponding direct current (DC) level. Here, the signal is rectified by means of a diode and converted to a DC level which varies in amplitude according to the magnitude of the received signal. The voltage components detected by signal detecting means 41 are supplied to time constant controlling means 42.
Time constant controller 42 receives the detected signal from signal detecting means 41 and removes the AC component included in the detected signal, to thereby produce a pure DC component and simultaneously provide a predetermined time response function characteristic. Time constant controlling means 42 includes a resistor R and a capacitor C and the time-response characteristic is determined by the RC time-constant. Since the gain is controlled at all times and whenever the amplitude of the detected signal changes, if the output signal of receiving amplifier 44 undergoes an inordinate amount of change, the change is buffered with a predetermined duration of time response (attack time). Accordingly, the signal at receiving signal output S2 is prevented from being changed erratically, and thereby receiving amplifier 44 can precisely produce a signal having the desired amplitude.
Control voltage amplifying means 43 receives the output of time-constant controlling means 42 and changes the output to desired gain control voltage S3. Gain control voltage S3 is then supplied to the aforementioned gain controlling node of receiving amplifier 44. The voltage level detected at signal detecting means 41 and time-constant controlling means 42 is determined according to the output of receiving amplifier 44, but may differ from the voltage level for controlling the desired gain of the gain voltage controlling node of receiving amplifier 44. Thus, control voltage amplifying means 43 offsets the difference. The control voltage amplifying means 43 also controls the direction of voltage change detected from the automatic gain controller to be consistent with the gain controlling direction of receiving amplifier 44 unless both of those directions are identical. In more detail, when the voltage detected from the automatic gain controller is in proportion to the amplitude of receiving signal S1 (i.e., receiving signal S1 diminishes and accordingly the detected DC current diminishes, or vice versa), if the receiving amplifier 44 operates in such a manner that the overall gain thereof becomes smaller when voltage S3 supplied to the gain controlling node becomes smaller, or, conversely, the overall gain becomes greater when voltage S3 supplied to the gain controlling node becomes greater, the output of time-constant controlling means 42 cannot be used as just being amplified, and thus, the output is inverted and then amplified by control voltage amplifier 43, and supplied to the gain controlling node of receiving amplifier 44.
The conventional automatic gain controller is classified as a high-speed automatic gain controller or a low-speed automatic gain controller, according to the response time of time-constant controlling means 42. These two types of gain controllers are selectively used according to design criteria.
FIG. 2 is a graphic diagram showing the change of voltage signals S3 according to time in high-speed response and low-speed response in the conventional automatic gain controller. Referring to FIG. 2, the low-speed automatic gain controller which sets a slow response time in time-constant controller 42 (FIG. 1) cannot cope actively with the quick change in receiving signal input S1 since the transition to the predetermined control voltage is carried out at a low speed. However, the low-speed automatic gain controller has a slower transient response characteristic to reach the predetermined voltage, so that it exhibits a more stable operation characteristic after the control voltage reaches its predetermined level, and thereby acts as a buffer for minute instantaneous changes in the received signal. Accordingly, the controller will not operate poorly, and output signal S2 of receiving amplifier 44 can be stabilized.
On the contrary, the high-speed automatic gain controller sets a fast response time in time-constant controlling means 42. Here, the controller can cope actively with a quick change of the receiving signal input S1 because the transition to a predetermined control voltage is performed at a fast speed, e, and the control voltage cannot be stabilized at the predetermined voltage level. Moreover, since the high-speed automatic gain controller has a greater tendency to control the gain for minute instantaneous changes of the received signal, and the output signal S2 of receiving amplifier 44 cannot be stable.
Accordingly, when the conventional automatic gain controller, to cope with the changes of the input signal, performs a transition from one control voltage level to a higher control voltage level, the response time characteristic of time-constant controlling means 42 is determined according to the charge reaching capacitor C via resistor R and its output load. In addition, when the gain controller performs a transition from a higher control voltage level to a lower one to cope with level changes in the input signal, the response time of time-constant controlling means 42 is determined according to the capacitance of capacitor C and the output load of the automatic voltage controller, such as the internal resistance of control voltage amplifier 43 or the control voltage node of receiving amplifier 44.
The conventional automatic gain controller shows a longer response time for discharging than for charging, since the load of the automatic gain controller. i.e., the DC resistance of the gain control voltage node of receiving amplifier 44, is greater than input resistor R. In other words, when the gain control voltage changes from a lower level to a higher level, to cope with the change of the input signal, the rise time is mainly determined according to the response time characteristic of time-constant controller 42. On the contrary, if the change in the received input causes the gain control voltage to change from a higher level to a lower level, the fall time is determined according to the output load of the automatic gain controller. Therefore, the fall time is longer than the rise time.
Accordingly, the transition from a lower control voltage level to a higher control voltage level shortens gain control time. However, a transition from a higher control voltage level to a lower control voltage level lengthens gain control time, which slows the response time of receiving amplifier 44.
To improve certain drawbacks due to the difference between the rise time and fall time in the prior art, it has been suggested that an additional load resistor be installed at the output of time-constant controlling means 42 (FIG. 1). This method is intended to shorten the rise time by intentionally discharging the charge accumulated in time-constant controlling means 42, but the response time is reduced by the additional resistor, and the rise time is also shortened, and thereby the control voltage becomes unstable at a predetermined voltage level.